1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) for internally imaging an object to be examined on the basis of a magnetic resonance phenomenon of nuclear spins of the object, particularly, to an MRI (magnetic resonance imaging) system and an MR (magnetic resonance) method capable of acquiring artery/vein visually separated images of the object without using a contrast medium.
2. Description of the Related Art
Magnetic resonance imaging is based on an imaging technique for magnetically exciting nuclear spins of an object located in a static magnetic field by applying a radio-frequency (RF) signal of a Larmor frequency and reconstructing an image from MR signals induced by the excitation.
For clinically obtaining blood flow images of the pulmonary field or abdomen of a patient by magnetic resonance imaging, MR angiography has been put in practical use, in which a contrast medium is injected into the object to highlight blood flows. However, this contrast MR angiography needs an invasive treatment to inject the contrast medium. First of all, mental and physical burdens on patients become large. Second, examination cost of the contrast MR angiography is still expensive. Third, there are some cases where a contrast medium cannot be injected into patients due to patient's physical characteristics.
In cases the contrast medium cannot be injected or is not used, imaging techniques, such as time-of-flight (TOF) and phase contrast (PC) techniques are used alternatively.
The time-of-flight method and phase contrast techniques utilize an effect of flows such as blood flows. The effect of flows is attributed to either of two natures possessed by spins in motion. One is that spins simply move their positions due to flows, while the other results from phase shifts of transverse magnetization caused when spins move in a gradient field. The nature of the position movement is used for the TOF technique and the nature of the phase shifts is used for the phase contrast technique.
However, when the TOF technique or phase contrast technique is used for obtaining MR images of a patient's pulmonary field or abdomen which depict flows of large vessels, such as the aorta, in their superior-inferior directions, it is required to scan slices located vertically to the flowing direction. That is, axial images should be acquired with a slice direction of those axial images set to the superior-inferior direction. Thus, in the case that two-dimensional slice imaging is performed to acquire such axial images, it is impossible to obtain an image in which blood flows are directly reflected. Three-dimensional image data spatially containing blood flows are therefore needed, but the number of slices increases which will cause an entire imaging time to be longer.
A novel MR imaging technique, known as an FBI (Fresh Blood Imaging) technique, has been proposed to overcome the foregoing inconveniences. In MR imaging on the FBI technique, an optimum time delayed from an R-wave of an ECG signal is predetermined, and ECG-synchronized MR scanning is performed at the delay time, thus well tracing a fresh and stable high-velocity blood flow ejected from the heart every appearance of the R-wave. In the FBI technique, three-dimensional scanning is additionally performed under the condition that signal intensities from parenchyma are actively suppressed by employing imaging conditions that include setting of a shorter repetition time TR (this causes the longitudinal relaxation time of parenchyma at rest to be insufficient) and applying an IR (Inversion Recovery) pulse or fat-suppression pulse (i.e., suppressing signals to be emanated from fat), thereby the blood flow being depicted. This eliminates the necessity of using a contrast medium, and blood flow images can be provided within a relatively shorter scan time.
For obtaining artery/vein visually separated blood flow images by using the FBI technique, a three-dimensional scan should be performed two times at different ECG-synchronized timings, and two sets of three-dimensional echo data acquired by the two-time three-dimensional scans or two sets of three-dimensional image data individually formed from the two sets of three-dimensional echo data should undergo weighted subtraction between the two sets of data.
In other words, even when the FBI technique is used, there remain some drawbacks that should be resolved. One drawback is that a longer scan time is still needed in total, because a three-dimensional scan should be performed two times. Another is that registration may be mistaken if the position of a patient's body moves between two times of scans, which is apt to deteriorate quality of blood flow images which will be produced by the subtraction.
On the other hand, the foregoing TOF and phase contrast techniques are based on the effect of flows of fluid such as blood. Although depending on the characteristics of an MRI system, it is general that either of the TOF or phase contrast method depicts only blood flows whose flowing speed is 2 to 3 cm/s or more. Blood flowing slower than this speed is scarcely detected. For example, peripheral veins, lymphatic vessels, CSF (cerebrospinal fluid), pancreatic duct, and others of a patient (human being) are slower in flow speed, and their flow speeds are approximately 1 cm/s or lower in general. Additionally, there may occur influence of positional shifts due to heartbeats, it was almost impossible to detect such slower-speed fluid flows by the conventional techniques.